Coating liquid for gate insulating film, gate insulating film and organic transistor

ABSTRACT

To provide a coating fluid for a gate insulating film, which can be baked at a low temperature of at most 180° C.; a gate insulating film having excellent solvent resistance and further having good characteristics in e.g. specific resistance or semiconductor mobility; and an organic transistor employing the gate insulating film. 
     A coating fluid for a gate insulating film, which comprises a polyimide obtainable by cyclodehydration of a polyamic acid having repeating units of a specific structure, a gate insulating film employing the coating fluid, and the organic transistor employing the gate insulating film.

TECHNICAL FIELD

The present invention relates to a coating fluid for a gate insulatingfilm suitable for lowering a temperature in a process for producing anorganic transistor, a gate insulating film formed from the coatingfluid, and further an organic transistor prepared from the gateinsulating film.

BACKGROUND ART

At present, as a substrate for a flexible device such as an electronicpaper, a plastic substrate made of e.g. a polycarbonate or apolyethylene terephthalate has been studied, but there is a problem thatsuch a substrate is likely to be slightly expanded/shrunk when heated,and accordingly it is acutely desired to improve the heat resistance. Onthe other hand, in order to reduce a thermal stress applied to a plasticsubstrate, it has been studied to lower the temperature in a process forproducing an organic transistor. One of process steps requiring thehighest temperature in the production of an organic transistor, is astep for forming a gate insulating film, in which the temperature isdesired to be lowered.

As a process for forming a gate insulating film at a lower temperature,e.g. a process for anodic oxidation of the surface of gate electrode(Patent Document 1) or a chemical vapor deposition process (PatentDocument 2) has been proposed, but such a process is cumbersome, andtherefore it is desired to employ a material which can readily be formedinto a film by coating, such as spin coating or printing method.

On the other hand, as an example of preparing a gate insulating film bycoating, firstly, an example may be mentioned, in which poly-4(vinylphenol) and poly(melamine-formaldehyde) are cured at 200° C. (Non-PatentDocument 1). However, in this example, the baking temperature is high ata level of 200° C., and at such a temperature, the influence over theheat-expansion or shrinkage of a plastic substrate will be remarkable,whereby it becomes difficult to prepare an electronic paper having finepixels. Further, another example may be mentioned, in which a polyimideprecursor curable at a low-temperature is baked at 180° C. (Non-PatentDocument 2). However, this example mentions a polyimide simply anddiscloses no specific structures of the polyimide. Further, a leakcurrent density is disclosed to be at least 1×10⁻⁹ A/cm² at 2 MV/cm, andthe insulation properties are still insufficient.

Further, Patent Document 3 discloses a polyimide obtained by imidationof a polyimide precursor made of cyclobutene tetracarboxylic aciddianhydride and a specific diamine having a hexafluoropropylidene groupin its molecule, discloses that it is possible to obtain a polyimidefilm having excellent transparency not only in the visible portion butalso in the ultraviolet portion even in the baking at a high temperatureof from 270 to 350° C., and discloses that such a polyimide film issuitably used for e.g. a protective film for a liquid crystal displaydevice or a semiconductor device, an insulating film or an opticalwaveguide material for optical communication. Further, in Examplesthereof, a polyimide precursor obtained from2,2′-bis(3-methyl-4-aminophenyl)hexafluoropropane and cyclobutanetetracarboxylic acid dianhydride, is baked at 300° C. to form apolyimide film. However, Patent Document 3 is silent about formation ofa polyimide film by a low-temperature baking suitable for a gateinsulating film, and therefore there is no disclosure at all which maysuggest a structure of a polyamide precursor or its relation with e.g.film-forming conditions, for obtaining a polyimide film having a goodfilm quality by a low-temperature baking.

Patent Document 1: JP-A-2003-258260

Patent Document 2: JP-A-2004-72029

Patent Document 3: W02000/22049

Non-Patent Document 1: “Journal of Applied Physics (J. Appl. Phys.)”Vol. 93, No. 5, 1 Mar. 2003, p. 2997-2981

Non-Patent Document 2: “Applied Physics Letters (Appl. Phys. Lett.)”Vol. 84, No. 19, 10 May 2004, p. 3789-3791

DISCLOSURE OF THE INVENTION Objects To Be Accomplished By the Invention

An object to be accomplished by the present invention is to make itpossible for a gate insulating film in an organic transistor excellentin properties, to facilitate film-formation by coating and to be bakedat a low temperature.

Accordingly, it is an object of the present invention to provide acoating fluid for a gate insulating film, which can be baked at atemperature of at most 180° C. in consideration of the heat resistanceof a plastic substrate; and a gate insulating film which can simply beformed by coating and further be baked at a low temperature,specifically, at a temperature of at most 180° C. in consideration ofthe heat resistance of a plastic substrate, further which is excellentin the solvent resistance to a solvent to be used at the time oflamination of an organic semiconductor on an upper layer of a gateinsulating film in a case where such lamination is required when thedevice-preparation process is carried out entirely by coating, andfurthermore which has excellent properties from the viewpoint of thespecific resistance, semiconductor mobility, etc. Further, it is anobject of the present invention to provide an organic transistoremploying the gate insulating film.

Means To Accomplish the Objects

In order to accomplish high specific resistance by a low-temperaturebaking, it is considered to be the best means to use a material mademainly of a soluble polyimide requiring no thermal imidation. However,since the solubility tends to be low if the imidated ratio becomes high,it is necessary to use a polyimide excellent in the solubility so as toaccomplish high solubility even in a high imidated ratio.

Further, if a high-boiling point solvent is used as a solvent to let apolyimide dissolve therein, it is necessary to carry out baking at ahigh temperature since it is required to carry out baking at almost thesame temperature as the boiling point so as to completely volatilize thesolvent, and therefore the polyimide is desired to be dissolved in alow-boiling point solvent.

Further, in addition, it is necessary to consider the balance betweenthe solubility in a low-boiling point solvent and the resistance to asolvent at the time of coating an organic semiconductor.

The present invention has been made under such circumstances, andprovides the following.

-   1. A coating fluid for a gate insulating film, which comprises a    polyimide obtainable by cyclodehydration of a polyamic acid having    repeating units represented by the following formula (1), and which    can be baked at a temperature of at most 180° C., preferably at most    150° C.-   2. A gate insulating film made of a polyimide film, wherein the    polyimide film is a film obtained by applying and baking a solution    containing an organic solvent-soluble polyimide, and further the    organic solvent-soluble polyimide is a polyimide obtained by    cyclodehydration of a polyamic acid having repeating units    represented by the following formula (1), and the baking temperature    for the polyimide film is at most 180° C., preferably at most 150°    C.,

wherein A is a tetravalent organic group, B₁ is at least one bivalentorganic group selected from the structures represented by the followingformulae (2) to (9), and B₂ is a bivalent organic group other than thefollowing formulae (2) to (9):

(wherein each R⁵ independently is a hydrogen, a methyl group or atrifluoromethyl group),

-   each of b1 and b2 represents a compositional ratio, and b1 and b2    have a relationship of 0.5≦(b1/(b1+b2))≦1 in ratio (mol).-   3. In the formula (1), A is preferably a tetravalent organic group    having an alicyclic structure.-   4. The tetravalent organic group having an alicyclic structure is    preferably at least one member selected from the group consisting of    the following formulae (10) to (14):

(in the formula (10), each of R¹, R², R³ and R⁴ which are independent ofone another, is hydrogen, fluorine or a C₁₋₄ organic group.)

-   5. In the formula (1), B₁ preferably represents the above formula    (2), (4), (6) or (8).-   6. In the formula (1), R₅ in B₁ preferably represents a methyl group    or a trifluoromethyl group.-   7. The baking temperature for the polyimide film is at most 150° C.-   8. The organic solvent-soluble polyimide has an imidated ratio of at    least 50%.-   9. The solvent in the solution containing an organic solvent-soluble    polyimide has a boiling point of at most 200° C.-   10. The organic transistor of the present invention preferably    employs such a gate insulating film.

Effect of the Invention

The gate insulating film of the present invention has advantages thatthe specific resistance is very high and a gate leak current isextremely low. Further, according to the coating fluid for a gateinsulating film of the present invention, it is possible to obtain ahigh-quality insulating film at a baking temperature of at most 180° C.,and therefore it is possible to apply the film also to a plasticsubstrate, whereby it is possible to achieve a low-temperature processfor an organic transistor in which a gate leak current is low. In such acase, it is possible to form a film even by baking at 150° C. byselecting a low-boiling point solvent.

Further, the amount of degas can be reduced and a long-term reliabilityof an organic transistor is also excellent. Further, the resistance to asolvent such as xylene, trichlorobenzene and trimethylbenzene issecured, and therefore it is possible to form an organic semiconductorlayer by coating.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic cross-sectional view of an organic transistorillustrating an application example of the gate insulating film of thepresent invention.

FIG. 2 is a schematic cross-sectional view of an organic transistorillustrating an application example of the gate insulating film of thepresent invention.

FIG. 3 is a graph showing degas intensity, at a molecular weight of 16,of a thin film obtained from the solution of each of PI-1 (EXAMPLE) andPI-4 (COMPARATIVE EXAMPLE).

FIG. 4 is a graph showing degas intensity, at a molecular weight of 18,of a thin film obtained from the solution of each of PI-1 (EXAMPLE) andPI-4 (COMPARATIVE EXAMPLE).

FIG. 5 is a graph showing degas intensity, at a molecular weight of 44,of a thin film obtained from the solution of each of PI-1 (EXAMPLE) andPI-4 (COMPARATIVE EXAMPLE).

FIG. 6 is a graph showing a relationship between a current density andan electric field of a thin film obtained from the solution of each ofPI-1 (EXAMPLE), PI-2 (EXAMPLE), PI-3 (EXAMPLE) and PI-4 (COMPARATIVEEXAMPLE).

FIG. 7 is a graph showing a relationship between a drain current and agate voltage of an organic transistor using a thin film obtained fromthe solution of each of PI-1 (EXAMPLE) and PI-4 (COMPARATIVE EXAMPLE),as a gate insulating film.

MEANINGS OF SYMBOLS

1: Substrate

2: Gate Electrode

3: Gate Insulating Film

4: Source Electrode and Drain Electrode

5: Organic Semiconductor Layer

BEST MODE FOR CARRYING OUT THE INVENTION

The gate insulating film of the present invention is made of a polyimidefilm, and the polyimide film is a film obtained by applying and baking asolution containing an organic solvent-soluble polyimide obtained bycyclodehydration of a polyamic acid having repeating units representedby the following formula (1), as a coating fluid for a gate insulatingfilm,

wherein B₁ is at least one bivalent organic group selected from thefollowing formulae (2) to (9), and B₂ is a bivalent organic group otherthan the following formulae (2) to (9):

wherein each R⁵ independently is hydrogen, a methyl group or atrifluoromethyl group. In the formulae, two R⁵s are usually the same,but may be different.

The feature of the structures of the formulae (2) to (9) for B₁ in theformula (1) lies in that two phenyl groups being present so as to haveone carbon atom or oxygen atom therebetween, are bonded to a polymerchain at 3,3′- or 3,4′-positions.

A polyimide having such bivalent organic groups has excellentsolubility, has solubility in a low-boiling point solvent even when theimidated ratio is high, and further has high insulation properties.

In the case of bonding at 3,3′-positions, the solubility will be higher,such being preferred.

The substituent (R⁵) in the above phenyl group is a hydrogen atom, amethyl group or a trifluoromethyl group, but a methyl group or atrifluoromethyl group is preferred for the purpose of increasing thesolubility of the polyimide, and a methyl group is more preferred whenelectrical characteristics of the gate insulating film is alsoconsidered.

A group connecting two phenyl groups is an isopropylidene group, ahexafluoroisopropylidene group, a methylene group or an ether group.From the viewpoint of the solubility, a hexafluoroisopropylidene groupis most preferred, and an isopropylidene group is then preferred. Fromthe viewpoint of the electrical characteristics of the gate insulatingfilm, an isopropylidene group or a methylene group is preferred.

For B₁ in the formula (1), the structure of the formulae (2) to (9) maybe one type or a mixture of plural types.

In the formula (1), each of b1 and b2 represents a compositional ratio,and b1 and b2 have a relationship of 0.5≦(b1/(b1+b2))≦1 in ratio (mol).Namely, the proportion of the structures of the formulae (2) to (9) asB₁ occupied in the total of B₁ and B₂, is preferably from 50 to 100 mol%.

The structure of B₂ occupying the residual 0 to 50 mol % may be onehaving an alicyclic structure, one having an aromatic ring structure,one having an aliphatic structure, one containing a heteroatom or anappropriate combination thereof, and thus may be not particularlylimited so long as it is a bivalent organic group other than theformulae (2) to (9) Further, such a structure may be one type or amixture of plural types. The structure and the ratio of such a portioncontributes to fine adjustment of the solubility of the polyimide orvarious properties of a gate insulating film, and thereforedetermination may be made by considering such properties.

Specific examples may be the following B-1 to B-69.

Among them, B-2, B-5, B-7, B-8, B-10, B-11, B-13, B-14, B-17, B-28,B-30, B-34, B-35, B-36, B-43, B-48, B-49, B-54, B-55 to B-69 may, forexample, be mentioned as ones for increasing the solubility of thepolyimide.

In the formula (1), A is a tetravalent organic group, but the structureof A is not particularly limited. The structure of A may be one type ora mixture of plural types.

Specific examples of A may be the following, such as alicyclic oneshaving alicyclic structures (A-1 to A-24), an aliphatic one (A-25),aromatic ones having all of four connecting bonds bonded to an aromaticgroup (A-26 to A-36), or others (A-37 to A-46).

Among them, the one having an alicyclic structure is preferred since thespecific resistance is high when formed into a polyimide film and thesolubility is also excellent, and the structures shown in the followingformulae (10) to (14) are particularly preferred.

In the formula (10), each of R¹, R², R³ and R⁴ which are independent ofone another, is hydrogen, fluorine or a C₁₋₄ organic group (for example,an alkyl group such as a methyl group or an ethyl group, or afluorinated substituent thereof). They may mutually be the same ordifferent.

A polyamic acid may be obtained by reacting a tetracarboxylicdianhydride with a diamine. In the reaction, by using the followingdiamine for 50 to 100 mol % of the diamine component, it is possible toobtain a polyamic acid having the repeating units represented by theformula (1).

In the above formulae, each R⁵ independently is hydrogen, a methyl groupor a trifluoromethyl group, and is the same as defined in the formulae(2) to (9).

In a case where A in the formula (1) gives a polyamic acid as astructure of the above formulae (10) to (14), the following compoundsmay be used as a tetracarboxylic acid dianhydride.

Here, each of R¹ , R², R³ and R⁴ which are independent of one another,is hydrogen, fluorine or a C₁₋₄ organic group, and is the same asdefined in the formula (10).

As a polymerization reaction to obtain a polyamic acid, a method ofmixing a tetracarboxylic dianhydride component and a diamine componentin an organic solvent is simple.

The method for mixing the tetracarboxylic dianhydride component and thediamine component in an organic solvent may, for example, be a methodwherein a solution having the diamine component dispersed or dissolvedin an organic solvent, is stirred, and the tetracarboxylic dianhydridecomponent is added as it is or as dispersed or dissolved in an organicsolvent, a method wherein inversely, the diamine component is added to asolution having the tetracarboxylic dianhydride component dispersed ordissolved in an organic solvent, or a method wherein the tetracarboxylicdianhydride component and the diamine component are alternately added.Further, in a case where the tetracarboxylic dianhydride component orthe diamine component is composed of plural types of compounds, suchplural types of the component may be subjected to polymerizationreaction in a preliminarily mixed state or may be separately andsequentially subjected to the polymerization reaction.

The temperature for the polymerization reaction of the tetracarboxylicdianhydride component with the diamine component in an organic solventis usually from −20 to 150° C., preferably from 0 to 80° C. As thetemperature becomes high, the polymerization reaction will be completedquickly, but if the temperature is too high, a polyamic acid having ahigh molecular weight may not sometimes be obtainable.

Further, the polymerization reaction can be carried out at an optionalconcentration, but if the concentration is too low, a polymer having ahigh molecular weight tends to be hardly obtainable, and if theconcentration is too high, the viscosity of the reaction solution tendsto be too high, and uniform stirring tends to be difficult. Accordingly,it is preferably from 1 to 50 mass %, more preferably from 5 to 30 mass%. At the initial stage of the polymerization reaction, thepolymerization reaction may be carried out at a high concentration, andthen, an organic solvent may be added.

The organic solvent to be used for the above reaction is notparticularly limited so long as it is capable of dissolving the formedpolyamic acid. Specific examples thereof may be N,N-dimethylformamide,N,N-dimethylacetamide, N-methyl-2-pyrolidone, N-methylcaprolactam,dimethylsulfoxide, tetramethylurea, pyridine, dimethylsulfone,hexamethylsulfoxide and γ-butyrolactone. They may be used alone or incombination as a mixture. Further, even a solvent which does notdissolve a polyamic acid, may be used as mixed to the above solventwithin such a range that the formed polyamic acid will not precipitate.Further, moisture in the reaction system tends to impair thepolymerization reaction and is likely to cause hydrolysis of the formedpolyamic acid. Therefore, it is preferred to employ an organic solventwhich has been dried to remove water. It is further preferred that theinterior of the reaction system is kept to be a nitrogen atmosphere, andit is further preferred to carry out the reaction while bubblingnitrogen in the solvent in the reaction system.

The ratio of the tetracarboxylic dianhydride component to the diaminecomponent to be used for the polymerization reaction for a polyamic acidis preferably from 1:0.8 to 1:1.2 by molar ratio. As this molar ratio isclose to 1:1, the molecular weight of the obtainable polyamic acidbecomes large.

As the cyclodehydration (imidation) reaction to obtain a solublepolyimide, thermal imidation of directly heating the solution of apolyamic acid obtainable as described above, as it is, or chemicalimidation of adding a catalyst to the solution of a polyamic acid, iscommon, but the chemical imidation wherein the imidation reactionproceeds at a relatively low temperature, is preferred, since a decreasein the molecular weight of the obtainable soluble polyimide is lesslikely to occur.

The chemical imidation may be carried out by stirring the polyamic acidin an organic solvent in the presence of a basic catalyst and an acidanhydride for from 1 to 100 hours.

The basic catalyst may, for example, be pyridine, triethylamine,trimethylamine, tributylamine or trioctylamine. Among them, pyridine ispreferred, since it has a proper basicity to let the reaction proceed.

The acid anhydride may, for example, be acetic anhydride, trimelliticanhydride or pyromellitic anhydride. Among them, acetic anhydride ispreferred, since purification of the obtained polyimide will be easyafter completion of the imidation.

As the organic solvent, it is possible to use a solvent which is used atthe time of the above-described polymerization reaction for a polyamicacid.

A solution of the soluble polyimide obtained as the above may be usedfor preparation of a gate insulating film, as it is, but is preferablyused after a polyimide is recovered/washed by the following procedure.

A reaction solution is put into a poor solvent being stirred toreprecipitate a polyimide. The poor solvent to be used at that time isnot particularly limited, and it may, for example, be methanol, hexane,heptane, ethanol, toluene and water. The polyimide obtained by thereprecipitation may be filtered and recovered and then washed with theabove poor solvent, and then dried at room temperature or under heatingunder atmospheric pressure or reduced pressure to obtain a powder. Whenan operation of further dissolving this powder in a good solvent,followed by reprecipitation is repeated from 2 to 10 times, wherebyimpurities in the polymer become less. Further, as the poor solvent insuch a case, it is preferred to employ at least three poor solvents suchas alcohols, ketones and hydrocarbons, whereby the purificationefficiency will be more increased.

The molecular weight of the polyamic acid to be used in the presentinvention is preferably from 2,000 to 200,000, more preferably from5,000 to 50,000 by weight average molecular weight from the viewpoint ofthe handling efficiency, the solubility in a solvent, the solventresistance to a nonpolar solvent and the stability of characteristicswhen formed into a film. Such a molecular weight is most preferably from10,000 to 50,000. The molecular weight is one obtained by means of GPC(gel permeation chromatography).

The imidated ratio of the soluble polyimide to be used in the presentinvention, may not necessarily be 100%.

There is a case where the solubility of a polyimide becomes insufficientif the imidated ratio is high, and in such a case, the imidated ratiomay be lowered.

However, if amic acid is remained, there is a case where water isproduced by cyclodehydration during heating, and the imidated ratio isdesirably high. Also from the viewpoint of high insulation properties,the imidated ratio is preferably high.

Accordingly, the imidated ratio of the soluble polyimide in the presentinvention is preferably at least 50%, more preferably at least 80%,particularly preferably at least 90%. The imidated ratio is one obtainedin such a manner that the polyimide is dissolved in d₆-DMSO(dimethylsulfoxide-d₆), and ¹H-NMR was measured, whereupon the ratio ofremaining amic groups to a theoretical value of amic groups not beingimidated at all is obtained from the integrated values of proton peaks,and all of amic groups disappeared is calculated as one changed to imidegroups.

The solution of the soluble polyimide as a coating fluid for a gateinsulating film is prepared as follows.

The soluble polyimide obtained as described above is dissolved in anoptimum solvent. The soluble polyimide may be used alone or incombination as a mixture of two or more of them.

The solvent for a coating fluid is not particularly limited so long asit is possible to dissolve the polyimide, but is preferably one having alow boiling point for the purpose of reducing a solvent remained in apolyimide film.

The lower the amount of the residual solvent is, the more the long-termreliability of an organic transistor improves.

The boiling point is preferably at most 200° C., more preferably at most180° C., furthermore preferably at most 160° C., most preferably at most150° C.

Further, in a case of an amide type polar solvent, such a solvent islikely to remain in a film even with a low boiling point, due to e.g.interaction with imide groups of a soluble polyimide or remaining amicacid groups without being imidated, and e.g. a glycol type solvent, alactate type solvent or a ketone type solvent is more preferred.Specific examples of such solvents include ethyl cellosolve (135° C.),butyl cellosolve (171° C.), ethylene glycol monomethyl ether acetate(156° C.), ethyl carbitol (193° C.), ethyl carbitol acetate and ethyleneglycol (196 to 198° C.); propylene glycol derivatives such as propyleneglycol monomethyl ether (119° C.), 1-ethoxy-2-propanol (132° C.),1-butoxy-2-propanol (133° C.), propylene glycol diacetate (190 to 191°C.), propylene glycol-1-monomethyl ether-2-acetate (146° C.), propyleneglycol1-monoethyl ether-2-acetate (158° C.), dipropylene glycol methylether (190° C.) and 2-(2-ethoxypropoxy)propanol (198° C.); lactic acidderivatives such as methyl lactate (144° C.), ethyl lactate (154° C.),n-propyl lactate, n-butyl lactate (185 to 187° C.) and isoamyl lactate;and acetone (56° C.), methyl-n-butyl ketone (144° C.), methyl-n-amylketone (152° C.) and cyclohexanone (156° C.) (here, values inparentheses represent boiling points of solvents, respectively.)Further, they may be used alone or in combination as a mixture of two ormore of them for securing the flatness of the coating film, forimproving the wettability of the substrate with the coating fluid or forthe purpose of e.g. adjusting the surface tension, polarity and boilingpoint of the coating fluid.

The concentration of the coating fluid is not particularly limited, butis preferably from 0.1 to 30 mass %, more preferably from 40 to 20 mass%, as a polymer component. Such a concentration may optionally be setdepending upon specifications of a coating device or a thickness of afilm to be obtained.

To the coating fluid, an additive such as a coupling agent may be addedfor the purpose of improving the adhesion between the polyimide film andthe substrate.

Specific examples of such a coupling agent include

-   3-aminopropyltrimethoxysilane,-   3-aminopropyltriethoxysilane,-   2-aminopropyltrimethoxysilane,-   2-aminopropyltriethoxysilane,-   N-(2-aminoethyl)-3-aminopropyltrimethoxysilane,-   N-(2-aminoethyl)-3-aminopropylmethyldimethoxysilane,-   3-ureidepropyltrimethoxysilane,-   3-ureidepropyltriethoxysilane,-   N-ethoxycarbonyl-3-aminopropyltrimethoxysilane,-   N-ethoxycarbonyl-3-aminopropyltriethoxysilane,-   N-trimethoxysilyl propyltriethylene triamine,-   N-triethoxysilyl propyltriethylene triamine,-   10-trimethoxysilyl-1,4,7-triazadecane,-   10-triethoxysilyl-1,4,7-triazadecane,-   9-trimethoxysilyl-3,6-diazanonyl acetate,-   9-triethoxysilyl-3,6-diazanonyl acetate,-   N-benzyl-3-aminopropyltrimethoxysilane,-   N-benzyl-3-aminopropyltriethoxysilane,-   N-phenyl-3-aminopropyltrimethoxysilane,-   N-phenyl-3-aminopropyltriethoxysilane,-   N-bis(oxyethylene)-3-aminopropyltrimethoxysilane,-   N-bis(oxyethylene)-3-aminopropyltriethoxysilane,-   ethylene glycol diglycidyl ether, polyethylene glycol diglycidyl    ether, propylene glycol diglycidyl ether,-   tripropylene glycol diglycidyl ether, polypropylene glycol    diglycidyl ether, neopentyl glycol diglycidyl ether, 1,6-hexanediol    diglycidyl ether, glycerol diglycidyl ether, 2,2-dibromoneopentyl    glycol diglycidyl ether, 1,3,5,6-tetraglycidyl-2,4-hexanediol,    N,N,N′,N′-tetraglycidyl-m-xylenediamine,    1,3-bis(N,N-diglycidylaminomethyl)cyclohexane and    N,N,N′,N′-tetraglycidyl-4,4′-diaminodiphenylmethane.

The content of such a functional silane-containing compound or an epoxygroup-containing compound is preferably from 0.1 to 30 mass %, morepreferably from 1 to 20 mass %, based on the total polymer mass in thecoating fluid.

The solution of the soluble polyimide is applied and baked as follows.

The solution of the soluble polyimide may be applied by a dippingmethod, a spin coating method, a transfer printing method, a rollcoating method, an ink jetting method, a spraying method or abrush-coating method, and uniform film forming is possible by therespective methods.

The baking method after the coating fluid is applied on the substrate isnot particularly limited, but it can be carried out by means of a hotplate or an oven in a suitable atmosphere i.e. in atmospheric air, aninert gas such as nitrogen or in vacuum.

The baking temperature is at most 180° C., preferably at most 150° C. inorder to evaporate the solvent and from the viewpoint of coating with aplastic substrate. The lower limit of the baking temperature is notparticularly limited, but is usually about 40° C. from the viewpoint ofthe evaporation of the solvent. Further, the baking time may usually beabout 0.5 to 5 hours. The baking may be carried out by at least two-steptemperature change. By baking step-by-step, it is possible to furtherincrease the uniformity of the coating film.

In a case where the imidated ratio of a soluble polyimide contained inthe coating fluid is less than 100%, the imidated ratio may be increasedby this baking.

The gate insulating film of the present invention is a polyimide filmobtained as mentioned above. If the gate insulating film is too thin,dielectric breakdown occurs in low electric fields, whereby such a filmdoes not operate as a transistor, and if it is too thick, high voltageis required to operate a transistor, and therefore the thickness ispreferably from 5 to 5,000 nm, more preferably from 50 to 1,000 nm, mostpreferably from 200 to 600 nm.

In a case where it is impossible to obtain a polyimide film having adesired thickness by a single coating operation, the coating operationmay be repeated a plurality of times.

FIG. 1 and FIG. 2 show examples of construction of an organic transistorusing the gate insulating film of the present invention.

As shown in the figures, in the organic transistor of the presentinvention, the gate electrode 2 is formed on the substrate 1, and thegate electrode 2 is covered with the gate insulating film 3 of thepresent invention. In the example of FIG. 1, the source electrode 4 andthe drain electrode 4 are placed on the gate insulating film 3, and theorganic semiconductor layer 5 is formed so as to cover them.

On the other hand, in the example of FIG. 2, the organic semiconductorlayer 5 is formed on the gate insulating film 3, and on the organicsemiconductor layer 5, the source electrode 4 and the drain electrode 4are placed.

Such a construction is not limited to the above examples, and theorganic transistor of the present invention is not particularly limitedso long as the gate insulating film of the present invention is used forthe organic transistor.

As a substrate to be used for the organic transistor of the presentinvention, plastics such as polycarbonate and polyethylene terephthalateare suitable for the reason that the mechanical flexibility isexcellent.

The gate electrode, the source electrode and the drain electrode may,for example, be made of a metal such as gold, silver, copper, aluminumor calcium, an inorganic material such as carbon black, a fullerene orcarbon nanotube; or an organic π conjugate polymer such aspolythiophene, polyaniline, polypyrrole, polyfluorene or derivativesthereof.

Such electrode materials may be used alone or in combination with aplurality of them, and different materials may be used for the gateelectrode, the source electrode and the drain electrode, respectively.

As a method for forming such electrodes, e.g. vacuum deposition orsputtering is usually employed. In the case of a nanometal ink or anorganic π conjugate polymer, it is possible to form an electrode bycoating such as spin coating, spray coating, printing or ink jetting,such being preferred.

At the time of forming an electrode by coating, as a solvent for ananometal ink or an organic π conjugate polymer, water or a variety ofalcohols are preferred since such a solvent can reduce damages to thegate insulating film of the present invention. Further, a polar solventsuch as N,N-dimethylformamide, N,N-dimethylacetamide, 2-pyrrolidone,N-methyl-2-pyrrolidone, N-ethyl-2-pyrrolidone, N-vinyl-2-pyrrolidone,N-methylcaprolactam, dimethylsulfoxide or tetramethylurea, is alsopreferred from the viewpoint of excellent solubility in an organic πconjugate polymer, but such a polar solvent is preferably used within arange of reducing damages to the gate insulating film of the presentinvention.

As an organic semiconductor material, pentacene, oligothiophene,triphenylamine, polythiophene or derivatives thereof may, for example,be mentioned, and as a film-forming method, spin coating, spray coating,printing or ink jetting may, for example, be mentioned. At that time, asolvent for the organic semiconductor is not particularly limited solong as it is possible to dissolve or uniformly disperse it, and it ispossible to reduce damages to the gate insulating film of the presentinvention.

For example, xylene, trichlorobenzene or trimethylbenzene may bementioned.

Now, the present invention will be described in further detail withreference to Examples and Comparative Examples, but it should beunderstood that the present invention is by no means limited thereto.

EXAMPLE 1

Firstly, Preparation Examples of the soluble polyimide of the presentinvention are given together with Comparative Preparation Example.

In this Example, the molecular weight of a soluble polyimide wasmeasured by a GPC (gel permeation chromatography) apparatus (SSC-7200,manufactured by Senshu Scientific co., ltd.). At that time, the columntemperature of a GPC column (KD-803/KD-805, manufactured by SHOWA DENKOK.K.) was 50° C., an eluant to be used was N,N-dimethylformamide (asadditives, 30 mmol/L of lithium bromide monohydrate (LiBr.H₂O), 30mmol/L of phosphoric anhydride crystal (o-phosphoric acid), and 10 ml/Lof tetrahydrofuran (THF) were contained), and the flow rate was 1.0ml/min.

Standard samples for preparing calibration curves to be used, were TSKstandard polyethylene oxides (molecular weights: about 900,000, 150,000,100,000, 30,000) manufactured by TOSOH CORPORATION, and polyethyleneglycols (molecular weights: about 12,000, 4,000, 1,000) manufactured byPolymer Laboratories Ltd.

Further, for the imidated ratio of a soluble polyimide, the polyimidewas dissolved in d₆-DMSO (dimethylsulfoxide-d₆), and ¹H-NMR wasmeasured, whereupon the ratio of amic groups remaining without beingimidated was obtained from the integrated values of proton peaks, andthe imidated ratio was calculated.

PREPARATION EXAMPLE 1 Preparation of Soluble Polyimide (PI-1)

In a nitrogen stream, into a 200 ml four-necked flask, 18.45 g (0.051mol) of 2,2-bis(3-amino-4-methylphenyl)hexafluoropropane was put anddissolved in 75.92 g of N-methyl-2-pyrrolidone (hereinafter referred tosimply as “NMP”) and then, 9.48 g (0.048 mol) of 1,2,3,4-cyclobutanetetracarboxylic dianhydride was added. The mixture was stirred at roomtemperature for 8 hours to carry out a polymerization reaction. Theobtained polyamic acid solution was diluted with NMP to 10 mass %. Tosuch a solution, 26 g of acetic anhydride and 16.1 g of pyridine wereadded as imidation catalysts, followed by reacting at room temperaturefor 30 minutes and further reacting at 40° C. for 90 minutes to obtain apolyimide solution. This solution was put into a large amount of a mixedsolvent of methanol and water, and the obtained white precipitate wascollected by filtration and dried to obtain a white polyimide powder.This polyimide powder was confirmed to be 95% imidated by ¹H-NMR. 4 g ofthis powder was dissolved in 46 g of propylene glycol monomethyl etherto obtain a 8 mass % solution of the polyimide (PI-1). The numberaverage molecular weight (Mn) and the weight average molecular weight(Mw) of the polyimide (PI-1) obtained were Mn=8,300 and Mw=16,900,respectively.

PREPARATION EXAMPLE 2 Preparation of Soluble Polyimide (PI-2)

In a nitrogen stream, into a 200 mL four-necked flask, 2.79 g (0.0068mol) of 2,2-bis[4-(4-aminophenoxy)phenyl]propane and 9.85 g (0.027 mol)of 2,2-bis(3-amino-4-methylphenyl)hexafluoropropane were put anddissolved in 75.92 g of NMP, and then 6.33 g (0.032 mol) of1,2,3,4-cyclobutane tetracarboxylic dianhydride was added. The mixturewas stirred at room temperature for 8 hours to carry out apolymerization reaction. The obtained polyamic acid solution was dilutedwith NMP to 5 mass %. To this solution, 12.8 g of acetic anhydride and9.9 g of pyridine were added as imidation catalysts, followed byreacting at 50° C. for 3 hours to obtain a polyimide solution. Thissolution was put into a large amount of a mixed solution of methanol andwater, and the white precipitate obtained was collected by filtrationand dried to obtain a white polyimide powder. This polyimide powder wasconfirmed to be 95% imidated by ¹H-NMR. 4 g of this powder was dissolvedin 46 g of propylene glycol monomethyl ether to obtain a 8 mass %solution of the polyimide (PI-2). The number average molecular weight(Mn) and the weight average molecular weight (Mw) of the polyimide(PI-2) obtained were Mn=8,400 and Mw=15,100, respectively.

PREPARATION EXAMPLE 3 Preparation of Soluble Polyimide (PI-3)

In a nitrogen stream, into a 200 mL four-necked flask, 5.58 g (0.014mol) of 2,2-bis[4-(4-aminophenoxy)phenyl]propane and 7.39 g (0.020 mol)of 2,2-bis(3-amino-4-methylphenyl)hexafluoropropane were put anddissolved in 76.7 g of NMP, and then 6.20 g (0.032 mol) of1,2,3,4-cyclobutane tetracarboxylic dianhydride was added. The mixturewas stirred at room temperature for 8 hours to carry out apolymerization reaction. The polyamic acid solution obtained was dilutedwith NMP to 5 mass %. To this solution, 14.8 g of acetic anhydride and11.5 g of pyridine were added as imidation catalysts, followed by areaction at 50° C. for 3 hours to obtain a polyimide solution. Thissolution was put into a large amount of methanol, and the whiteprecipitate obtained was collected by filtration and dried to obtain awhite polyimide powder. This polyimide powder was confirmed to be 95%imidated by ¹H-NMR. 4 g of this powder was dissolved in 46 g ofpropylene glycol monomethyl ether to obtain a 8 mass % solution of thepolyimide (PI-3). The number average molecular weight (Mn) and theweight average molecular weight (Mw) of the polyimide (PI-3) obtainedwere Mn=8,900 and Mw=16,100, respectively.

COMPARATIVE PREPARATION EXAMPLE 1 Preparation of Polyamic Acid (PI-4)

In a nitrogen stream, into a 200 mL four-necked flask, 8.01 g (0.04 mol)of 4,4′-diaminodiphenyl ether was put and dissolved in 91.9 g of NMP,and then 8.20 g (0.038 mol) of pyromelletic dianhydride was added. Themixture was stirred at 23° C. for two hours to carry out apolymerization reaction and further diluted with NMP to obtain a 6 mass% solution of the polyamic acid (PI-4). The number average molecularweight (Mn) and the weight average molecular weight (Mw) of the polyamicacid (PI-4) were Mn=11,200 and Mw=24,300, respectively.

Now, e.g. the structures of polyimides and polyamic acids obtained areshown below. Among them, PI-1 is one obtained from the formula (1) inwhich B₁=formula (4) [R⁵=CH₃] and A=formula (10) [R¹, R², R³, R⁴=H].

Imidated Molecular Molecular n/(n + m) ratio weight (Mw) weight (Mn)P1-1 1 95% 16,900 8,300 P1-2 0.8 95% 15,100 8,400 P1-3 0.6 95% 16,1008,900 Comparative polymer

Imidated Molecular Molecular ratio weight (Mw) weight (Mn) 0% 24,30011,200

EXAMPLE 2

In order to investigate the solubility of the polyimides (PI-1) to(PI-3) prepared in Preparation Examples 1 to 3 in a low-boiling pointsolvent, an experiment was carried out under the following conditions byusing propylene glycol monomethyl ether (PGME), propylene glycolmonomethyl ether acetate (PGMEA), methyl n-amyl ketone (MAK),cyclohexanone (CH) and ethyl lactate (EL).

Into a round flask, any one of the low-boiling point solvents was put,and a powdery PI-1 was put thereto to be 10 mass %, and stirring wascarried out for 12 hours while the flask was heated in an oil bath sothat the temperature of the solvent would be 50° C. PI-2 and PI-3 werealso subjected to experiments under the same conditions.

The results are shown in Table 1. Further, the boiling points ofsolvents used are shown in Table 2. The solubility was visuallyevaluated, and in Table 1, “◯” represents a case where an unmelted solidcontent was not observed, and “X” represents a case where a is largeamount of an unmelted solid content was observed. PI-1 and PI-2 showedgood solubility in all of the solvents. Further, PI-3 was hardlydissolved in solvents other than cyclohexanone, but at least 9 mass % ofPI-3 was dissolved in cyclohexanone, and it was confirmed that there isno practical problem in film-formation of a gate insulating film.

It becomes apparent that the polyimide of the present invention hasexcellent solubility also in a low boiling point solvent.

TABLE 1 PGME PGME MAK CH EL PI-1 ◯ ◯ ◯ ◯ ◯ PI-2 ◯ ◯ ◯ ◯ ◯ PI-3 X X X 9.1mass % X

TABLE 2 Boiling point (° C.) PGME Propylene Glycol Monomethyl Ether118-119 PGMEA Propylene Glycol Monomethyl Ether 145-146 Acetate MAKMethyl n-Amyl Ketone 149-150 CH Cyclohexanone 155 EL Ethyl Lactate 154

EXAMPLE 3

In this Example, the amount of degas from a polyimide film was measuredby using TDS (Thermal Desorption Spectroscopy) system (WA1000S,manufactured by ESCO Ltd.), at a temperature range of from 50 to 300° C.and at a temperature-rising rate of 1° C./sec.

Degas Evaluation of PI-1 (Example)

On an Si wafer (thickness: 0.5 mm), the solution of PI-1 prepared inPreparation Example 1 was dropped by a syringe provided with a 0.2μm-perforated filter and applied by a spin coating method. Then, inatmospheric air, it was heated on a hot plate of 80° C. for 5 minutes toevaporate the organic solvent and then such an Si wafer was cut into asquare of 1 cm×1 cm, followed by baking for 60 minutes on a hot plate of150° C. to obtain a polyimide film having a thickness of about 200 nm.By using TDS, the amount of degas from the thin film made of polyimideand polyamic acid was evaluated at Mw=16, 18 and 44. The evaluationresults are shown in FIGS. 3 to 5. The degas from the polyimide film washardly observed, and therefore it is considered there is an extremelylow possibility that PI-1 brings about the deterioration of performanceof an organic transistor due to degas.

Degas Evaluation of PI-4 (Comparative Example)

By using a solution of PI-4 prepared in Comparative Preparative Example1, a polyimide film was formed in the same manner as in the aboveExample except that the film thickness was changed to 220 nm, and theamount of degas was measured by using TDS, whereupon an extremely largeamount of degas was observed at all of Mw=16, 18 and 44. The evaluationresults are shown in FIGS. 3 to 5. When Mw=16, an extremely large amountof degas was observed on end after the vicinity of 100° C., andtherefore an adverse effect on device characteristics is concerned.Further, even when Mw=18 and 44, a peak of degas was observed in thevicinity of 160° C. Especially, H₂O shown as Mw=18 is considered toaccelerate deterioration of an organic semiconductor layer, andtherefore it is considered there is extremely high possibility that PI-4brings about a problem in a long-term reliability and thermal stabilityof an organic transistor.

EXAMPLE 4

In this Example, the film thickness of polyimide was calculated in sucha manner that a film was partly removed by a cutter knife, and adifference in level was measured by using a fully automatic microfiguremeasuring instrument (ET4000A, manufactured by Kosaka Laboratory Ltd.),at a measurement power of 10 μN at a sweep rate of 0.05 mm/s.

Solvent Resistance of PI-1 (Example)

In order to investigate the solvent resistance of PI-1 prepared inPreparation Example 1, by using xylene, trichlorobenzene ortrimethylbenzene to be usually used for dissolving an organicsemiconductor polymer, experiments were conducted under the followingconditions.

On an ITO-coated glass substrate (2.5 cm×2.5 cm, thickness: 0.7 mm), thesolution of PI-1 prepared in Preparation Example 1 was dropped by asyringe provided with a 0.2 μm-perforated filter and applied by a spincoating method. Then it was heated on a hot plate of 80° C. for 5minutes to evaporate the organic solvent and then baked on a hot plateof 150° C. for 60 minutes to obtain a polyimide film having a thicknessof 250 nm. Then, any one solvent of xylene, trichlorobenzene andtrimethylbenzene was put into 100 mL of a beaker and heated by using anoil bath so that the liquid temperature would be ₈₀° C., and a polyimidesubstrate prepared as the above was immersed in the solvent for 30minutes. Then, such a polyimide thin film substrate was dried by usingan air blower to evaluate the film remaining rate.

The film remaining rate was calculated from the ratio of the filmthickness before solvent treatment to the film thickness after solventtreatment. The results are shown in Table 3.

The film remaining rate of PI-1 is 104% to xylene, and thereforeswelling was slightly observed, but such a level is not practicallyproblematic. On the other hand, the film remaining rates totrichlorobenzene and trimethylbenzene are 101% and 100% respectively,and it was found that such a film is not influenced by the solvent atall. Further, no dissolution was observed in any of the solvents.Accordingly, it becomes apparent that the gate insulating film for anorganic transistor of the present invention is excellent in the solventresistance.

Solvent Resistance of PI-2 (Example)

In order to investigate the solvent resistance of PI-2 prepared inPreparation Example 2, an experiment was conducted under the sameconditions as in the above except that the film thickness of polyimidewas changed to 300 nm. The results are shown in Table 3.

The film remaining rate of PI-2 was 102% to xylene, and thereforeswelling was slightly observed, but such a level is not practicallyproblematic. The film remaining rates to trichlorobenzene andtrimethylbenzene were 101% and 100% respectively, and it was found thatsuch a film was not influenced by the solvent at all. Further, nodissolution was also observed in any of the solvents. Accordingly, itbecomes apparent that the gate insulating film for an organic transistorof the present invention is excellent in the solvent resistance.

The Solvent Resistance of PI-3 (Example)

In order to investigate the solvent resistance of PI-3 prepared inPreparation Example 3, an experiment was conducted under the sameconditions as in the above except that the film thickness of polyimidewas changed to 230 nm. The results are shown in Table 3.

The film remaining rates of PI-3 were 105% and 106% to xylene andtrichlorobenzene respectively, and therefore swelling was slightlyobserved, but such a level is not practically problematic. On the otherhand, it was found that the film remaining rate was 100% totrimethylbenzene, and it was found that such a film is not influenced bythe solvent at all. Further, no dissolution was also observed in any ofthe solvents. Accordingly, it becomes apparent that the gate insulatingfilm for an organic transistor of the present invention is excellent inthe solvent resistance.

TABLE 3 Film thickness of polyimide (nm) Before After Film remainingPolyimide Treating solvent treatment treatment rate (PI-1) Xylene 244.2253.3 104% Trichlorobenzene 245.5 248.9 101% Trimethylbenzene 246.8247.2 100% (PI-2) Xylene 293.1 299.3 102% Trichlorobenzene 296.9 300.9101% Trimethylbenzene 297.9 298.4 100% (PI-3) Xylene 229.6 241.9 105%Trichlorobenzene 228.8 242.3 106% Trimethylbenzene 229.8 230.3 100%

EXAMPLE 5

In this example, by using HP4156C (manufactured by Agilent K.K.) andAG-4311B (manufactured by Ando electric Co., Ltd.) at 100 KHz and 1 V,the specific resistance and a relative permittivity of a polyimide film(gate insulating film) were respectively measured in such a manner thatin order to remove the influence of an ambient humidity and an activematerial, and after completion of a device, it was rapidly moved into anitrogen gas atmosphere and left to stand for about 10 minutes until theatmosphere was stabilized, followed by the measurement.

Evaluation of Insulating Property of PI-1 (Example)

In order to investigate insulating property of PI-1 prepared inPreparation Example 1, the specific resistance of a thin film made ofpolyimide and polyamic acid was measured in accordance with thefollowing experimental procedure.

On an ITO-coated glass substrate (2.5 cm×2.5 cm, thickness: 0.7 mm), thesolution of PI-1 prepared in Preparation Example 1 was dropped by asyringe provided with a 0.2 μm-perforated filter and applied by a spincoating method. Then, it was heated on a hot plate of 80° C. for 5minutes to evaporate the organic solvent and then baked on a hot plateof 150° C. for 60 minutes to obtain a polyimide film having a thicknessof about 220 nm. Then, in order to achieve good contact between an ITOelectrode and a probe, the polyimide film was partly shaved to exposeITO, and an aluminium electrode having a diameter of 0.5 mm and athickness of 100 nm was laminated on the polyimide film and the ITO byusing a vacuum deposition apparatus. At that time the conditions ofvacuum deposition were such that the temperature would be roomtemperature, the vacuum degree would be at most 3×10⁻³ Pa and thealuminium deposition rate would be at most 0.3 nm/sec. Thus, suchelectrodes are formed on and under the polyimide thin film, whereby asample for measuring a specific resistance of a polyimide thin film wasprepared.

The specific resistance was measured in a nitrogen atmosphere, in whichthe voltage was set to 19 V so that the applied electric field would be1 MV/cm and the current density was measured upon expiration of 60seconds after the voltage was applied, whereupon the current value was0.029 to 0.070 pA. The specific resistance was calculated therefrom,whereupon the specific resistance was from 6.2×10¹⁵ to 1.5×10¹⁶Ω·cm, andsuch a film was found to have sufficient insulating property as a gateinsulating film for an organic transistor even when baked at 150° C.

Further, in order to investigate the relationship between a leak currentand an electric field more detail, a voltage was applied from 0 to 50 Vwhile stepwisely increased every 2 V for a retention time of 10 seconds,and a relationship of the current density and the electric field at thattime was plotted (FIG. 6). FIG. 6 shows that the current density wasmerely at most 10⁻¹⁰ A/cm² even when a high voltage of 50 V (2.3 MV/cm)was applied, and a very excellent characteristic were shown as a gateinsulating film for an organic transistor. Further, a specificpermittivity of such a polyimide film was 3.0.

Evaluation of Insulating Property of PI-2 (Example)

By using a solution of PI-2 prepared in Preparation Example 2, apolyimide film was formed by the same method as in the above except thatthe film thickness was changed to 210 nm, whereby an element formeasuring the specific resistance was prepared. The specific resistancewas measured under the same conditions as in the above conditions exceptthat the voltage was set at 21 V so that 1 MV/cm of an electric fieldwas applied on a thin film made of PI-2. The current flown in aninsulating film made of polyimide and polyamic acid obtained from asolution of PI-2 was from 0.024 to 0.066 pA, and the specific resistancecalculated therefrom was from 7.6×10 ¹⁵ to 2.1×10¹⁶Ω·cm. Even in a casewhere baking is carried out at 150° C., such a film had sufficientinsulating property for the use as a gate insulating film for an organictransistor.

Further, a relationship of the current density and the electric fieldwas plotted in the same manner as the above (FIG. 6), whereupon thecurrent density was found to be merely at most 10⁻¹⁰ A/cm² even when ahigh voltage was applied, and therefore very excellent characteristicswere shown as a gate insulating film for an organic transistor even whenbaking was carried out at 150° C.

Evaluation of Insulating Property of PI-3 (Example)

By using a solution of PI-3 prepared in Preparation Example 3, apolyimide film was formed in the method as in the above except that thefilm thickness was changed to 220 nm, whereby an element for measuringthe specific resistance was prepared. The specific resistance wasmeasured under the same conditions as in the above except that thevoltage was set at 22 V so that 1 MV/cm of the electric field wasapplied on a thin film made of PI-3. The current flown in an insulatingfilm made of polyimide and polyamic acid obtained from a solution ofPI-2 was from 0.020 to 0.086 pA, and the specific resistance calculatedtherefrom was from 5.7×10¹⁵ to 2.4×10¹⁶Ω·cm. Even when baking wascarried out at 150° C., very excellent characteristics were shown as agate insulating film for an organic transistor.

Further, a relationship of the current density and the voltage wasplotted in the same manner as the above (FIG. 6), whereupon the currentdensity was found to be merely at most 10⁻¹⁰ A/cm² even when a highvoltage was applied, even when the baking was carried out at 150° C.,very excellent characteristics were shown as a gate insulating film foran organic transistor.

Evaluation of Insulating Property of PI-4 (Comparative Example)

By using a solution of PI-4 prepared in Comparative Preparation Example1, a polyimide film was prepared in the same manner as in the aboveexcept that the film thickness was changed to 210 nm, whereby an elementfor measuring the specific resistance was prepared. The specificresistance was measured under the same conditions as in the above exceptthat the voltage was set at 22 V so that 1 MV/cm of the electric fieldwas applied on a thin film made of PI-4. The current flown in aninsulation film made of polyimide and polyamic acid, obtained from asolution of PI-4 was 7.0 pA, and the specific resistance calculatedtherefrom was 7.2×10¹³Ω·cm. In a case where an insulating film made ofpolyimide and polyamic acid, obtained from a solution of PI-4 was bakedat 150° C., such a film was found to be remarkably inferior in thespecific resistance as a film to be used as a gate insulating film foran organic transistor. Further, the specific permittivity of thepolyimide film was 3.6.

EXAMPLE 6

By using HP4156C (manufactured by Agilent K.K.), the electriccharacteristics of an organic transistor was measured in such a mannerthat in order to remove the influence of an ambient humidity and anactive material, after completion of an element, it was rapidly movedinto the vacuum state (vacuum degree: at most 5×10⁻² Pa) and then leftto stand for about 30 minutes, and then the electrical characteristicswere measured.

Evaluation of Electric Characteristics of Organic Transistor (Example)

In order to evaluate characteristics as a gate insulating film ofpolyimide, an organic transistor was prepared. On an ITO-coated glasssubstrate (2.5 cm×2.5 cm, thickness: 0.7 mm), the solution of PI-1prepared in Preparation Example 1 was dropped by a syringe provided witha 0.2 μm-perforated filter and applied by a spin coating method. Then,it was heated on a hot plate of 80° C. for 5 minutes to evaporate theorganic solvent and then baked on a hot plate of 150° C. for 60 minutesto obtain a polyimide film having a thickness of about 425 nm. Further,the capacitance C of the insulating film was calculated from thespecific permittivity and the film thickness of polyimide, and was foundto be 6.25×10⁻⁹ (F/cm²). Then, poly(3-hexylthiophene-2,5-diyl)(hereinafter, abbreviated as “P3HT”) containing at least 98.5% of an HTbond manufactured by Sigma-Aldrich Co., was sufficiently purified anddissolved in 1 mass % of xylene to prepare a coating fluid of P3HT. Sucha coating fluid was applied on polyimide to form a P3HT thin film. Sucha film-forming method was carried out by using a spin coating methodunder nitrogen atmosphere in an oxygen concentration of at most 0.5 ppm.Then, in order to completely evaporate a solvent, heating treatment wascarried out under a vacuum state at 100° C. for 90 minutes. Then, byusing a vacuum deposition apparatus, about 60 nm of gold was laminatedon a P3HT film to form a source electrode and a drain electrode having achannel length L of 90 μm and a channel width W of 2 mm. Further, theconditions during the vacuum deposition were such that a temperature wasroom temperature, the vacuum degree was at most 3×10⁻³ Pa, thedeposition rate of gold was at most 0.1 nm/sec. Such an organictransistor prepared was left to stand over night under a nitrogenatmosphere in an oxygen concentration of at most 0.5 ppm, and thenexposed in an atmosphere once so as to be mounted in a measurementapparatus, followed by evaluation of the electric characteristics.

By using a vacuum chamber, the electric characteristics were measuredwhile the vacuum degree was kept at 5×10⁻² Pa or lower. In order tomeasure modulation of a drain current to a gate voltage, at asource/drain voltage (V_(D)) of −60 V, the gate voltage (V_(G)) waschanged stepwisely every 2 V from +20 V to −60 V, and the value obtainedafter the voltage was maintained for three seconds until the current wassufficiently stabilized, was taken as a measurement value of the draincurrent. The characteristics (V_(G)-I_(D) characteristics) of a draincurrent to a gate voltage, measured in such a manner, are shown in FIG.7. By applying the gate voltage in the minus direction, a drasticincrease of the drain current I_(D) was observed, whereby P3HT wasconfirmed to be operated as a p-type semiconductor. Usually, it ispossible to represent the drain current I_(D) in a saturated state, asthe following formula. Namely, it is possible to obtain the mobility μof an organic semiconductor from the inclination of a graph when thesquare root of the absolute value of the drain current I_(D) was plottedon the ordinate and the gate voltage V_(G) was plotted on the abscissa.

I _(D) =WCμ(V _(G) −V _(T))²/2L

In the above formula, W represents a channel width of a transistor, L achannel length of a transistor, C a capacitance of a gate insulatingfilm, V_(T) threshold of a transistor, and μ a mobility. The mobility μof P3HT was calculated on the basis of the above formula, and was foundto be 9.1×10⁻⁴ cm²/Vs. Further, the threshold voltage V_(T) was 15 V,the current flown in an off-state of an organic transistor, i.e. theoff-current was 0.65 nA, and the ratio (on/off ratio) of on-state tooff-state was 580 (Table 4).

Thus, the gate insulating film of the present invention showedsufficiently excellent characteristics as an organic transistor.

TABLE 4 Insulating Off-current film Mobility On/off V_(T) (nA) nF/cm²Example 9.1 × 10⁻⁴ 580 15 0.65 6.25 Comparative 8.7 × 10⁻⁴ 15 Not 38.98.56 Example measurable

Evaluation of Electric Characteristics of Organic Transistor(Comparative Example)

By using a solution of PI-4 prepared in Comparative Preparation Example1, an organic transistor was prepared in the same manner as in the aboveexcept that the film thickness was changed to 370 nm, and the electriccharacteristics was measured. The results are shown in Table 7. Thecapacitance C of such an insulating film was calculated from thespecific permittivity and the film thickness of polyimide, and was foundto be 8.56×10⁻⁹ (F/cm²). The mobility of P3HT was 8.7×10⁻⁴ cm²/Vs, theoff-current was 38.9 nA, and the on/off ratio was 15. The thresholdvoltage was not measurable (Table 4) since it is impossible for anorganic transistor to reach complete off-state.

Thus, the organic transistor using PI-4 has a very large amount of leakcurrent and cannot reach normal off-state, and therefore it was foundthat such an organic transistor cannot be used as an organic transistor.

EXAMPLE 7 PREPARATION EXAMPLE 4 Preparation of Soluble Polyimide (PI-5)

In a nitrogen stream, into a 200 ml four-necked flask, 5.435 g (0.015mol) of 2,2-bis(3-amino-4-methylphenyl)hexafluoropropane was put anddissolved in 36.75 g of NMP, and then 3.753 g (0.015 mol) ofbicyclo[3.3.0]-octane-2,4,6,8-tetracarboxylic dianhydride was added. Themixture was stirred at 80° C. for 48 hours to carry out a polymerizationreaction. The polyamic acid solution obtained was diluted with NMP to 8mass %. To such a solution, 19.55 g of acetic anhydride and 9.09 g ofpyridine were added as imidation catalysts, followed by reacting at 100°C. for 3 hours to obtain a polyimide solution. This solution was putinto a large amount of a mixed solvent of water and methanol, and theobtained white precipitate was collected by filtration and dried toobtain a white polyimide powder containing repeating units representedby the following formula. This polyimide powder was confirmed to be 90%imidated by ¹H-NMR. The number average molecular weight (Mn) and theweight average molecular weight (Mw) of such a polyimide powder wereMn=11,400 and Mw=18,000, respectively.

PREPARATION EXAMPLE 5 Preparation of Soluble Polyimide (PI-6)

In a nitrogen stream, into a 200 mL four-necked flask, 2.029 g (0.0056mol) of 2,2-bis(3-amino-4-methylphenyl)hexafluoropropane was put anddissolved in 24.67 g of NMP, and then 3.36 g of (0.0112 mol) of3,4-dicarboxy-1,2,3,4-tetrahydro-1-naphthalene succinic acid dianhydridewas added, followed by stirring at 50° C. for 15 hours. And 5.045 g(0.0168 mol) of 3,4-dicarboxy-1,2,3,4-tetrahydro-1-naphthalene succinicacid dianhydride, 3.925 g (0.0196 mmol) of 4,4-diaminodiphenyl ether and1.055 g (0.0028 mol) of 1-octadecyloxy-2,4-diaminobenzene were addedthereto, followed by stirring at 50° C. for 24 hours to carry out apolymerization reaction. The polyamic acid solution obtained was dilutedwith NMP to 8 mass %. To this solution, 28.55 g of acetic anhydride and13.28 g of pyridine were added as imidation catalysts, followed by areaction at 50° C. for 3 hours to obtain a polyimide solution. Thissolution was put into a large amount of methanol, and the obtained whiteprecipitate was collected by filtration and dried to obtain a whitepolyimide powder containing repeating units represented by the followingformula. This polyimide powder was confirmed to be 95% imidated by¹H-NMR. The number average molecular weight (Mn) and the weight averagemolecular weight (Mw) of such a polyimide powder were Mn=13,500 andMw=28,200, respectively.

(Wherein the ratio of 1, k and j was represented by the followingformula. 1/(1+k+j)=0.2, and k/(1+k+j)=0.7)

PREPARATION EXAMPLE 6 Preparation of Soluble Polyimide (PI-7)

In a nitrogen stream, into a 200 mL four-necked flask, 2.79 g (0.00689mol) of 2,2-bis(3-amino-4-methylphenyl)hexafluoropropane and 9.85 g(0.0272 mol) of 2,2-bis(3-amino-4-methylphenyl)hexafluoropropane wereput and dissolved in 76.48 g of NMP and then 6.59 g (0.0337 mol) of1,2,3,4-cyclobutane tetracarboxylic dianhydride was added. The mixturewas stirred at room temperature for 8 hours to carry out apolymerization reaction. The obtained polyamic acid solution was dilutedwith NMP to 5 mass %. To this solution, 12.8 g of acetic anhydride and9.9 g of pyridine were added as imidation catalysts, followed by areaction at 50° C. for 3 hours to obtain a polyimide solution. Thissolution was put into a large amount of a mixed solution of methanol andwater, and the obtained white precipitate was collected by filtrationand dried to obtain a white polyimide powder containing repeating unitsrepresented by the following formula. This polyimide powder wasconfirmed to be 95% imidated by ¹H-NMR. The number average molecularweight (Mn) and the weight average molecular weight (Mw) of such apolyimide powder were Mn=24,000 and Mw=42,000, respectively.

(Wherein the ratio of n and m is represented by the following formula.n/(n+m)=0.8)

COMPARATIVE PREPARATION EXAMPLE 2 Preparation of Soluble Polyimide(PI-8)

In a nitrogen stream, into a 200 mL four-necked flask, 2.79 g (0.00689mol) of 2,2-bis(3-amino-4-methylphenyl)hexafluoropropane and 9.85 g(0.0272 mol) of 2,2-bis(3-amino-4-methylphenyl)hexafluoropropane wereput and dissolved in 76.48 g of NMP, and then 6.67 g (0.034 mol) of1,2,3,4-cyclobutane tetracarboxylic dianhydride was added. The mixturewas stirred at room temperature for 8 hours to carry out apolymerization reaction. The polyamic acid solution obtained was dilutedwith NMP to 5 mass %. To this solution, 12.8 g of acetic anhydride and9.9 g of pyridine were added as imidation catalysts, followed by areaction at 50° C. for 3 hours to obtain a polyimide solution. Thissolution was put into a large amount of a mixed solution of methanol andwater, and the obtained white precipitate was collected by filtrationand dried to obtain a white polyimide powder represented by thefollowing formula. This polyimide powder was confirmed to be 95%imidated by ¹H-NMR. The number average molecular weight (Mn) and theweight average molecular weight (Mw) of the obtained polyimide powderwere Mn=29,000 and Mw=60,000, respectively.

(Here the ratio of n and m is represented by the following formula.n/(n+m)=0.8)

EXAMPLE 8 Solvent Solubility of PI-5 (Example)

In order to investigate the solvent solubility of PI-5, by usingpropylene glycol monomethyl ether (PGME), propylene glycol monomethylether acetate (PGMEA), cyclohexanone and γ-butyrolactone, experimentswere carried out under the following conditions.

Into a round flask, any one of low-boiling point solvent was put, and 10mass % of a powdery PI-6 was charged thereinto, followed by stirring for3 hours while heating the solvent in an oil bath so that the solventtemperature would be 50° C.

PI-5 had a good solubility in any of the solvents.

Solvent Solubility of PI-6 (Example)

In order to investigate a solvent solubility of PI-6, by usingcyclohexanone and γ-butyrolactone, experiments were carried out underthe following conditions.

Into a round flask, any one of low-boiling point solvents was put, and10 mass % of a powdery PI-6 was charged thereinto, followed by stirringfor 3 hours while heating the solvent in an oil bath so that the solventtemperature would be 50° C.

PI-6 showed good solubility in any of the solvents.

Solvent Solubility of PI-7 (Example)

In order to investigate the solvent solubility of PI-7, cyclohexanonewas put into a round flask, and 10 mass % of a powdery PI-7 was chargedthereinto, followed by stirring for 3 hours while heating the solvent inan oil bath so that the solvent temperature would be 50° C.

PI-7 showed a good solubility in cyclohexanone.

Solvent Solubility of PI-8 (Comparative Example)

In order to investigate the solvent solubility of PI-8, by usingpropylene glycol monomethyl ether (PGME), propylene glycol monomethylether acetate (PGMEA) and cyclohexanone, experiments were carried outunder the following conditions.

Into a round flask, any one of low-boiling point solvents was put, and10 mass % of a powdery PI-8 was charged thereinto, followed by stirringfor 3 hours while heating the solvent in an oil bath so that the solventtemperature would be 50° C.

PI-8 was insoluble in all of the solvents.

The evaluation results of the above solubility were summarized in Table5 (provided that blanks represents is “not evaluated”).

TABLE 5 Imidated PGME PGMEA CH GBL Mn Mw ratio Ex. PI-5 ◯ ◯ ◯ ◯ 11,40018,000 90% Ex. PI-6 ◯ ◯ 13,500 28,200 95% Ex. PI-7 ◯ 24,000 42,000 95%Comp. PI-8 X X X 29,000 60,000 95% Ex.

EXAMPLE 9 Solvent Resistance of PI-5 (Example)

In order to investigate the solvent resistance of PI-5 to xylene, apolyimide film having a thickness of 307 nm was formed in the samemanner as in Example 4, and immersed in a xylene solvent heated to 50°C. for 5 minutes. Then, a solvent was dried on a hot plate of 180° C. tomeasure the film remaining rate. The film remaining rate of a polyimidefilm made of PI-5 was 100%, and therefore good solvent resistance wasshown.

INDUSTRIAL APPLICABILITY

By the gate insulating film of the present invention, it is possible tolower a temperature in a production process of an organic transistor andto provide an inexpensive and high-performance organic transistor.

The entire disclosure of Japanese Patent Application No. 2006-143716filed on May 24, 2006 including specification, claims, drawings andsummary is incorporated herein by reference in its entirety.

1. A coating fluid for a gate insulating film, which comprises apolyimide obtainable by cyclodehydration of a polyamic acid havingrepeating units represented by the following formula (1):

wherein A is a tetravalent organic group, B₁ is at least one bivalentorganic group selected from the following formulae (2) to (9), and B₂ isa bivalent organic group other than the following formulae (2) to (9):

(wherein each R⁵ independently is hydrogen, a methyl group or atrifluoromethyl group), each of b1 and b2 represents a compositionalratio, and b1 and b2 have a relationship of 0.5≦(b1/(b1+b2))≦1 in ratio(mol).
 2. A gate insulating film made of a polyimide film, wherein thepolyimide film is a film obtained by applying and baking a solutioncontaining an organic solvent-soluble polyimide, and further the organicsolvent-soluble polyimide is a polyimide obtained by cyclodehydration ofa polyamic acid having repeating units represented by the followingformula (1), and the baking temperature for the polyimide film is atmost 180° C.,

wherein A is a tetravalent organic group, B₁ is at least one bivalentorganic group selected from the following formulae (2) to (9), and B₂ isa bivalent organic group other than the following formulae (2) to (9):

(wherein each R⁵ independently is a hydrogen, a methyl group or atrifluoromethyl group), each of b1 and b2 represents a compositionalratio, and b1 and b2 have a relationship of 0.5≦(b1/(b1+b2))≦1 in ratio(mol).
 3. The gate insulating film according to claim 2, wherein in theformula (1), A is a tetravalent organic group having an alicyclicstructure.
 4. The gate insulating film according to claim 3, wherein thetetravalent organic group having an alicyclic structure is at least onemember selected from the group consisting of the following formulae (10)to (14):

(in the formula (10), each of R¹, R², R³ and R⁴ which are independent ofone another, is hydrogen, fluorine or a C₁₋₄ organic group.)
 5. The gateinsulating film according to any one of claims 2 to 4, wherein in theformula (1), B₁ represents the above formula (2), (4), (6) or (8). 6.The gate insulating film according to any one of claims 2 to 5, whereinin the formula (1), R₅ in B₁ represents a methyl group or atrifluoromethyl group.
 7. The gate insulating film according to any oneof claims 2 to 6, wherein the baking temperature for the polyimide filmis at most 150° C.
 8. The gate insulating film according to any one ofclaims 2 to 7, wherein the organic solvent-soluble polyimide has animidated ratio of at least 50%.
 9. The gate insulating film according toany one of claims 2 to 8, wherein the solvent in the solution containingan organic solvent-soluble polyimide has a boiling point of at most 200°C.
 10. An organic transistor employing the gate insulating film asdefined in any one of claims 2 to 9.